1. Field of the Invention
The present invention relates to an ink, an ink-jet recording process, and and instrument making use of the ink. More particularly it relates to an ink, an ink-jet recording process, a recording unit, an ink-jet recording apparatus and an ink cartridge that are used in plain paper-adapted ink-jet recording, capable of making a record on non-coated paper such as wood free paper, medium wood free paper or bond paper, i.e., what is called plain paper.
2. Related Background Art
Inks used in the plain paper-adapted ink-jet recording are required to have mainly the following four performances:
(1) Fixing performance: Evaluated on the basis of the time by which the ink prints formed on a recording paper is apparently dried and the paper is no longer stained when rubbed with fingers. The shorter the time is, the more preferable.
(2) OD (optical density): Reflection density of an image recorded by printing on a recording paper. In general, a higher OD is obtained when a dye does not penetrate into the inner part of the paper and remains on the surface in a greater proportion. Such a higher OD is preferable.
(3) Print quality level: print quality level is deemed better when a dot recorded by printing is closer to a perfect circle. Conventional inks give a dot with notches because of the influence of the fiber of paper, when printed on plain paper.
(4) Anti-stick properties. Properties of preventing ink from being evaporated to solidification in an ink-jet recording head. The less solidification that is caused, the more preferable.
It is difficult to produce an ink capable of satisfying all the performances, since most of these characteristics conflict with each other. For example, the fixing performance and the anti-stick properties. When a stress is laid on the fixing performance, it is the condition of good ink that the ink is readily evaporated. On the other hand, when a stress is laid on the anti-stick properties, the situation is quite the reverse, where it is the condition of good ink that the ink is not readily evaporated, bringing about a great inconsistency.
The fixing performance and the OD also have a similar inconsistency. In order to improve the fixing performance, the penetrability of ink into recording paper may be improved. Such an improvement, however, results in a sinking of ink to the depth direction of the paper. Penetration of ink into the inner part of the paper results in a lowering of the OD because of the light scattering caused by the fiber at the paper surface.
The fixing performance and the print quality level also bring about a similar inconsistency. Namely, an improvement in the penetrability of ink into recording paper results in an improvement in the fixing performance. Since, however, paper has a non-uniform structure (unevenness or fibrous structure), there is a difference in the penetration depth (the distance of penetration.) Use of an ink that readily penetrates increases the difference in this penetration depth, resulting in the absence of formation of a round dot but formation of a dot, with notches.
Thus, it has been hitherto very difficult to produce an ink capable of satisfying all of the four performances of ink, i.e., the fixing performance, OD, print quality level and anti-stick properties, with respect to the ink used in the plain paper-adapted ink-jet recording. Under such circumstances, there have been some attempts to improve inks.
Japanese Patent Application Laid-open No. 54-143305 discloses an example in which a polyhydric hydroxyalkyl derivative of an aromatic sulfonamide is used as a substance for improving the anti-stick properties of ink. In this example, however, the ink can achieve superior anti-stick properties, but has a low OD and a poor print quality level and fixing performance. In addition, this ink can not achieve a good initial ejection performance when ejected using a head for ink-jet recording. The initial ejection performance refers to the state of ejection of the ink that is ejected as the first shot after water has been evaporated from the nozzle tips during the stop of printing. The reason why the ink disclosed in the Japanese Patent Application Laid-open No. 54-143305 is not superior in this initial ejection performance is unclear, but is presumed to be that the hydroxyl groups of the polyhydric hydroxyalkyl of an aromatic sulfonamide form hydrogen bonds and hence the viscosity of evaporated ink is extremely increased. As an example of aromatic sulfonamides having a plurality of hydroxyl groups, an ink disclosed in Japanese Patent Application Laid-open No. 54-104933 also causes a poor initial ejection performance for the same reason.
Thus, none of the conventional inks have been able to completely satisfy the performances required as inks for ink-jet recording.